Fluid catalytic cracking process comprises of cracking heavier boiling range hydrocarbon streams e.g., vacuum gas oil and residues or mixture thereof in the presence of cracking catalyst at reactor outlet temperature of usually above 500° C. FCC products include hydrocarbons with carbon number 1 to 50 and hence cover entire boiling range starting from fuel gas to residue. FCC reactor product mixture is separated in a main fractionator by distillation.
In the conventional process of propylene and LPG recovery from FCC main column overhead product mixture, the gaseous fraction from the main fractionator condenser/separator is fed to a two stage compressor. The first stage discharge is partially condensed and cooled in inter-stage coolers. The resulting liquid and gaseous fractions are separated in inter-stage receiver. Second stage compressor discharge after combining with the liquid fraction from first-stage receiver is condensed and cooled in second stage high pressure condenser/coolers and received in a high pressure receiver cum separator.
The liquid fraction from high pressure receiver is fed to a de-ethanizer column or C2-stripper where ethane, ethylene and lighter material present in the feed are removed. The overhead vapours from stripper are recycled back to high pressure receiver via high pressure coolers. Bottom product of the stripper is fed to a Debutanizer column where propylene is obtained as a part of the overhead product and the bottoms product thus obtained is referred to as stabilized naphtha.
The gaseous fraction from high pressure receiver is supplied to an absorber. In the absorber, C3—C4 components present in the gaseous feed are preferentially absorbed by an absorber fluid also referred to as absorber oil or lean oil. Overhead liquid from the main fractionator (typically known as unstable naphtha) and debut bottoms liquid (typically known as stabilized naphtha) are commonly used as absorber oil. Typical, temperature of lean oil supplied to the absorber column is between 30 and 40° C. Side coolers are provided to remove heat of absorption from absorber oil. Rich absorber oil from absorber bottom is cooled and supplied to high pressure receiver from where it is fed to de-ethanizer column. Absorber overhead gases flows are further treated to recover any gasoline range material still present in the gas leaving the absorber.
It is found that the propylene recovery in the conventional process is limited up to 97 wt %, which is primarily due to the presence of significant quantity of propylene (>5 mol %) in the unstabilized naphtha stream. This retards the mass transfer of propylene from fuel gas to absorber oil.
U.S. Pat. No. 7,074,323 B2 describes a process to debottleneck the above described conventional process for gas concentration unit wherein unstabilized naphtha, a liquid fraction obtained by cooling the main fractionator overheads and subsequently separating the obtained gaseous and liquid fractions, is separated by distillation into a heavy boiling fraction (Initial boiling point 100-160° C.) and a lighter fraction (Final boiling point 10-160° C.). The lighter fraction after being cooled between 8 to about 25° C. is fed to the absorber while the heavier fraction is directly fed to the debutanizer. This reduces liquid and gas loads on absorber, stripper and debutanizer. However, the recovery of propylene is not much improved since the lighter fraction contains in fact higher percentage of propylene than the original cut before fractionation. Moreover the main objective of the patent was to reduce the load on C2 stripper and Debutanizer section rather than improving propylene recovery.
U.S. Pat. No. 3,893,905 by UOP describes a process to improve propylene recovery wherein a differential condenser rather than the conventional condenser and receiver is used to condense main fractionator overhead vapors for obtaining unstabilized naphtha fraction. Use of this differential condenser minimizes absorption of propylene and C4s in unstabilized naphtha, so that when used as lean oil in the absorber, it absorbs more propylene. The main concept here is to drain the liquid as soon as it is formed by condensation and not let this condensed liquid mix with propylene and lighters present in vapor phase. However, this requires heat exchangers with specially designed baffles which may not operate efficiently over a wide range of feed and other operating conditions and has not been proved in industry. The other option is to supply lean oil at lower temperatures to the absorber which can further reduce propylene and LPG content of the fuel gas. However, this requires chilling of the cooling water system which requires major investment. Another common practice to make the absorber oil leaner is by recycling more of debut bottoms i.e., stabilized naphtha to the absorber, since this recycle is free of propylene and LPG components. But this can not be done in units where any of the absorber, de-ethanizer or debutanizer columns are constrained due to vapor/liquid flooding or due to limited reboiling duties. In fact, in FCC unit with high propylene recovery, these columns are used to the fullest extent for achieving highest propylene separation. Also debut bottom recycle can not be increased in units where cooling duty of debut bottom recycle circuit is already limiting, since the resulting higher temperatures in the absorber will offset the benefit of using leaner oil for higher absorption.
From the above discussion, it is clear that there is a significant economic incentive to develop a FCC process for enhanced LPG and propylene recovery, which will improve the propylene separation from fuel gas beyond 97 wt % but without increasing load in any of existing columns or exchangers in gas concentration section of FCC. The present invention offers a way to enhance the absorption capacity of lean oil used in the absorber without increasing stabilized naphtha i.e. debut bottom recycle to the absorber. Alternately, the invention can be used to deconstrain absorber-stripper-debutanizer by decreasing debut bottom recycle to the absorber.